Problem: A swimming pool had $2.5$ million liters of water in it. Some water evaporated, and then the pool only had $2$ million liters of water in it. What percent of the water evaporated?
Explanation: Let's start by finding the amount of water that evaporated. $2.5\,\text{million L}-2\,\text{million L}=0.5\,\text{million L}$ What percent of $2.5$ million liters is $0.5$ million liters? We can rewrite that question as an equation. $\begin{array}{ccccc} {\text{what percent}}&\text{of}&{2.5\,\text{million L}}&\text{is}&0.5\,\text{million L}\\\\ {?\%}&\times &{2.5}&=&0.5 \end{array}$ Let's solve for the percent. $\begin{aligned} \dfrac{{?\%}\times {2.5}}{2.5}&=\dfrac{0.5}{2.5}\\\\ {?\%}&={0.2} \end{aligned}$ Percent means per hundred, so ${0.2}$ is equivalent to ${\dfrac{20}{100}}$ and also to ${20\%}$. $20\%$ of the water evaporated.